1. Field of the Invention
The present invention relates to an information recording medium, and also it is concerned with a process for preparing an information recording medium.
2. Related Background Art
Organic type optical recording mediums used in the technical field of optical discs or optical cards are provided on a substrate such as glass and plastics, with a heat mode recording layer comprising a spiropyran-containing coloring matter composition, an organic metal and so forth, wherein a beam of light for writing is irradiated on this recording layer to form a pit by removing a part of this recording layer by fusion decomposition or changing the structure of coloring matters, thus recording information in a high density. Regarding the optical recording mediums in which the information thus recorded is read out by detecting the pits with a beam of light for read-out, there have been hitherto proposed those having hollow structure. A typical cross-section thereof is shown in FIG. 1.
In FIG. 1, the numerals 1a and 1b each denote a substrate having a central hole and transparent to laser beams, and made of glass or plastics. The numerals 2a and 2b each denote a recording layer formed into a film on the substrate. In some instances, a very thin subbing layer is laminated beneath the recording layer containing the organic coloring matter that may undergo the fusion decomposition or coloring matter structural change owing to the laser beams, and an auxiliary layer such as a protective layer is laminated on the same. The two substrates 1a and 1b on which the filmy recording layer 2a or 2b was formed are stuck together (adhered) in such a manner that a hollow space 4 is held between them with spacers respectively interposed in the vicinity of an inner peripheral edge and an outer peripheral edge to control the thickness of the hollow space, by use of adhesives 3 and 3' coated in the shape of rings and serving as the spacers. Also well known is an optical information recording medium of air-sandwiched structure in which the information recording layer is provided only on one of the substrate.
As the adhesive used here, there have been developed those of thermosetting types, two-part liquid setting types, moisture curing types, hot-melt types, etc., but most preferred are ultraviolet curing type adhesives that can be quickly cured and have good workability.
However, it is very difficult to effect a 100% complete polymerization by the ultraviolet curing, and the remaining monomers may volatilize in the sealed hollow space to accelerate the deterioration of the recording layer by the interaction with the recording layer.
For this reason, Laid Open Patent Application No. 85643/1986, for example, proposes to add in the recording layer a compound capable of reacting with ultraviolet-curing monomers. However, adding in the recording layer the additive other than the coloring matter has caused the problems such that the sensitivity is lowered because of a decrease in the absorbance of the recording layer or the signal intensity ratio is lowered.
There has also been the problem that, because of a large degree of cure shrinkage of generally available polymerization type adhesives, the stress by strain may remain in the adhered part and there can be achieved no sufficient adhesion to a plastic substrate.